Process for the separation of chlorosilanes from gas streams

ABSTRACT

Chlorosilanes are continuously removed from a gas stream in an apparatus in which the gas stream is treated in a first stage with water vapor in the gas phase, and in a second stage with a liquid, aqueous phase.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is a process for the separation of chlorosilanesfrom gas streams, in which chlorosilane containing gas streams arebrought into contact with water vapor in a first stage and with liquidwater in a second stage.

2. Discussion of the Background

Chlorosilanes and hydrogen are formed in the reaction of silicon withhydrogen chloride. This reaction is termed the direct synthesis ofchlorosilanes. By cooling the reaction gases of this reaction, a liquidchlorosilane mixture may be condensed. However, even at condensationtemperatures far below 0° C., chlorosilanes which are liquid at roomtemperature, i.e., trichlorosilane (silicochloroform) andtetrachlorosilane (silicon tetrachloride), still have a considerablevapor pressure, and therefore significant amounts of these chlorosilanesmay remain in the hydrogen-rich waste gas. In addition, small amounts ofchlorosilanes which are gaseous at room temperature, i.e.,dichlorosilane and optionally monochlorosilane, are also formed in thereaction, and may be present in the waste gas. All of thesechlorosilanes must be separated from the hydrogen-rich waste gas beforethe waste gas may be burned, with or without energy recovery, or used insome other way.

EP 0 089 783 A2 describes a process for the treatment of liquid,chlorosilane-containing waste products or byproducts from thepreparation of silicones by hydrolysis of organochlorosilanes in anaqueous medium. In this process, the average Si—Cl functionality of thewaste products or byproducts is at least 2.8, and it is intended thatthe hydrolyzate be recovered in the form of a granulated gel. However,it has been shown that such gas-scrubbing processes simply transform awaste gas problem into a waste water problem, because gelatinousoligomers which are extremely difficult to filter remain in the washingwater.

EP 0 532 857 A1 describes a process for steam hydrolysis of the residuesproduced by a chlorosilane distillation process, which in its continuousembodiment is carried out at an initial temperature of at most 160° C.and at a final temperature of at least 170° C.

There is therefore a need for a continuous process in whichchlorosilanes can be removed simply and reliably from gas streams byhydrolysis, and the hydrolysis product may be produced in a form whichcan be easily disposed of without problems. Preferably, such a processshould largely avoid the formation of deposits and blockages in thehydrolysis apparatus so that long operating times are possible. Furtherobjects of the invention are obvious from the following description.

SUMMARY OF THE INVENTION

It has been found that chlorosilanes may be continuously and effectivelyremoved from gas streams if the gas streams are treated in two stages,first, in the gas phase with water vapor, for example steam, and second,with a liquid, aqueous phase.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, is a schematic diagram of an apparatus for carrying out theprocess of the present invention in its first embodiment.

FIG. 2, is a schematic diagram of an apparatus for carrying out theprocess of the present invention in its second embodiment.

FIG. 3, is a schematic diagram of an apparatus for carrying out theprocess of the present invention in its third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Chlorosilanes are obtained quantitatively in the form of readilyseparable solid hydrolysis products by treating chlorosilane containinggas streams with steam in the gas phase, then treating the gas streamwith a liquid, aqueous phase. It is assumed that in the gas phase, i.e.in the first stage of the process, hydrolysis of Si—Cl bonds commenceswith the initial formation of presumably gaseous substances, which aretermed primary hydrolysis products. The assumption that hydrolysis takesplace in the gas phase is supported by the generally known fact thatchlorosilanes are decomposed by atmospheric moisture to give SiO₂ andHCl. The assumption that the primary hydrolysis products are gaseous issupported by the fact that it is not possible to separate them asparticles, e.g. in cyclones. Even in the so-called precoating process,in which an additional layer of hydrolysis products are deposited onauxiliary materials, (e.g., limestone, kieselguhr or active carbon)before filtration, primary hydrolysis products cannot be successfullyseparated from the gaseous treatment mixture.

In the second stage of the process, the gas stream is treated with aliquid, aqueous phase, and the primary hydrolysis products produced inthe gas phase may act as condensation nuclei. A heterogeneous condensateis produced, consisting of predominantly solid hydrolysis products ofthe chlorosilanes and of the water resulting from condensation of thesteam and/or water supplied from other sources. The liquid, aqueousphase contains hydrogen chloride as well as dissolved monomeric,oligomeric or polymeric hydrolysis products (presumably monomeric,oligomeric or polymeric silicic acids). The solid or dissolvedhydrolysis products are termed secondary hydrolysis products.

The present process is particularly suited for the removal ofchlorosilanes from gas streams produced by the direct process for thepreparation of chlorosilanes, from silicon (ferrosilicon) and hydrogenchloride, in which chlorosilanes are condensed at low temperatures.Generally, such gas streams contain 3 to 60% by weight, preferably 5 to15% by weight, of chlorosilanes, which consist mainly of trichlorosilaneand, to a smaller extent, of tetrachlorosilane and dichlorosilane andoptionally traces of monochlorosilane. A considerable proportion of suchgas streams consists of hydrogen (hydrogen is the predominant componenton a molar basis) with minor, but still considerable, proportions ofhydrogen chloride and of other constituents, e.g. hydrogen sulfide. Ingeneral up to 40% by weight, and in particular 0.5 to 10% by weight, ofthe gas stream is hydrogen chloride and other constituents. Afterlow-temperature condensation, the gas streams generally have atemperature of −50 to −80° C. If desired, the cold gas stream may bepreheated to a temperature of up to 200° C. or more, before beingtreated by the chlorosilane separation process of the present invention.

First Embodiment

In the first embodiment of the process of the present invention, the gasstream containing chlorosilanes is first treated at a temperature of atleast 125° C., preferably 130 to 250° C., with independently suppliedsteam. The first embodiment differs in this respect from the secondembodiment, described below. Preferably, the gas stream is mixed withsuperheated steam, such that the mixed gas/steam stream has atemperature of at least 125° C., preferably 130 to 250° C. For a givensteam temperature, the amount of steam employed may be lower if the gasstream is preheated indirectly to a temperature of up to 200° C., e.g.to 40 to 200° C.

The steam serves as both a source of heat, as well as a reactant in thehydrolysis of the chlorosilanes. In the course of the hydrolysisreaction, hydrogen chloride is produced. Si—H bonds, however, remainunaffected. As discussed above, the chlorosilanes produced by hydrolysiswith steam in the first stage of the process afford a primary,presumably gaseous hydrolysis product. For the hydrolysis of thechlorosilanes, a minimum amount of steam is needed, namely 1 mol of H₂Oper equivalent of chlorine atoms. An excess of steam of from 1.2 to 300times, in particular of from 8 times to 250 times, the stoichiometricamount is advantageous in order that the hydrolysis, as well as theseparation of the silicon-containing hydrolysis products, be rapid andcomplete. This amount of steam is also preferred because adequateamounts of a liquid, aqueous phase are formed in the subsequentcondensation in the second stage of the process. In the course of thesecond stage of the process, the primary hydrolysis products oligomerizeor polymerize further to give readily separable secondary hydrolysisproducts. If steam of 140 to 260° C., and at a correspond pressure ofapproximately 4 to 50 bar, is used in an amount of 5 to 500 times,preferably 10 to 60 times the amount of chlorosilanes, by weight, thereis generally both sufficient heating capacity to establish a temperatureof >125° C., even in the case of nonpreheated gas streams, and forcomplete conversion of the chlorosilanes into readily separablesecondary hydrolysis products. These secondary hydrolysis products arein the form of a heterogeneous condensate, together with amounts ofwater which can still economically be evaporated in order to effectseparation of the secondary hydrolysis products.

The steam treatment times in the first stage can be very short, and maybe less than 1 sec, for example 0.0005 to 1.5 sec and preferably 0.001to 1.0 sec. These short treatment times correspond to high flow rates,for example from 5 to 30 m/sec. The treatment times for the treatmentwith the aqueous phase in the second stage are considerably longer. Ingeneral, the treatment time for the two stages together takes up between3 and 300 sec. The flow rates in the second stage are considerably lowerthan in the first stage and are, for example, only 0.001 to 0.1 times,preferably 0.002 to 0.01 times, the flow rate of the first stage.Depending on the volume of the cooling container in which the secondstage of the process proceeds, described below, the residence time ofthe components of the gas stream during this stage is correspondinglylonger.

In the first embodiment of the process according to the invention, thetreatment of the chlorosilane-containing gas stream with steam in thefirst stage advantageously takes place in a tubular reactor. The gasstream is preferably introduced into the interior of a steam stream inthe direction of gas flow, thereby reducing the formation of fixeddeposits on the tube wall.

In the second stage of the process, the gas stream emerges from thesteam treatment zone with approximately the same temperature establishedupon mixing the chlorosilane-containing gas stream with steam in thesteam treatment zone. The gas stream is then introduced into a coolingcontainer where it is cooled to a temperature below the dew point. As aresult, the steam condenses, and a heterogeneous condensate consistingof a liquid aqueous phase and the solid or dissolved secondaryhydrolysis products, separates. The liquid aqueous phase is stronglyacidic because of the hydrogen chloride formed by hydrolysis of thechlorosilanes, contained in the gas stream. The remaining gas stream isin contact, and thereby treated with, the resulting acidic liquidaqueous phase.

Heat is extracted from the gas stream and the heterogeneous condensatein the cooling container, by means of a cooling agent which flowsthrough a cooling jacket, through external or internal cooling coils, orsimilar devices. The cooling agent is preferably water at a temperatureof from 10 to 90° C.

An important feature of the cooling container is that it hasperpendicular or approximately perpendicular walls. By perpendicular, wemean that the walls form angles of −30 to +30°, advantageously from −15°to +15° and in particular of approximately 0°, with the vertical.Perpendicular or approximately perpendicular walls of this type assistthe heterogeneous condensates in falling from the walls to the bottom ofthe cooling container.

In a preferred embodiment of the first embodiment of the presentinvention, the mixture of a chlorosilane-containing gas stream and steamhaving a temperature of >125° C. is passed from above a verticallyoriented, cylindrical cooling container through a vertical orientedreaction tube, which projects centrally from above the cooling containerinto the lower third of the cooling container. The cooling container isequipped with a cooling jacket through which cooling water flows. Thecooling container may be subdivided into two cooling zones, a smallerupper cooling zone which is cooler than the larger, lower cooling zone.More of the steam is separated in the warmer lower zone than in thecooler upper zone. Owing to this subdivision of the cooling container,less cooling water is needed, which facilitates the later removal of thesecondary hydrolysis product from the aqueous phase of the heterogeneouscondensate. Alternatively, the gas stream may be treated with steam in atubular reactor completely outside the cooling container. The gas streammay then be directed tangentially into the cooling container, which thenhas only one cooling zone.

In the cooling container, the steam condenses with the formation ofdroplets, which as mentioned, possibly nucleate on the primaryhydrolysis products. These droplets are transported without furthertreatment and precipitate as a film on the cooled surfaces of the coolerareas of the cooling zone. The driving force for the transport of thedroplets is the flow caused by the decrease in volume of the gas streamresulting from condensation of the steam (Stefan flow).

If the internal walls of the cooling container and the surfaces of allinternal parts, such as cooling coils, are smooth, i.e. have nomacroscopic unevenness, cracks, grooves, lugs or other projections, theheterogeneous condensate slides particularly easily from the vertical orapproximately vertical surfaces, together with the secondary hydrolysisproduct formed, and collects at the bottom of the cooling container inthe form of an aqueous phase, on which a very water-rich, finely dividedsecondary hydrolysis product floats. Smooth inner walls or surfaces areall the more important the more the slope of the inner walls or surfacesis removed from 0° from the vertical. The nature of the materials fromthe which the cooling container and other internal parts, etc., areconstructed is less important than the condition of their surfaces.However, hydrophilic materials, such as glass, enamel and smoothlypolished hydrochloric acid-resistant steel, are preferred to morehydrophobic materials, which may include most polymers, such as, forexample, polytetrafluoroethylene. Polymers moreover have poorer heattransfer properties.

As mentioned above, the heterogeneous condensate at the bottom of thecooling container consists of water, on which a very water-rich, solidsecondary hydrolysis product of the chlorosilanes floats. In general,the solid hydrolysis product consists to 95.0 to 99.5% by weight ofincluded aqueous phase. The solid component is very finely divided orgelatinous and, just as in the previously discussed gas-scrubbingprocesses of the prior art, can only be separated from the water withdifficulty. Since the amount of water employed in the process of thepresent invention is, however, comparatively small, it can be removed inan economical manner by evaporation, thereby forming a nearly anhydrous,finely divided silica residue of low density, which can be disposed ofat low cost. Hydrogen chloride, which was a component of thechlorosilane-containing gas stream, and formed partly by hydrolysis ofthe chlorosilanes, remains dissolved in the aqueous phase. The aqueousphase furthermore contains dissolved hydrolysis products, presumablysilicic acid oligomers or polymers.

FIG. 1 is a schematic diagram of an apparatus for carrying out the firstembodiment of the process according to the present invention. At themixing site 1, the chlorosilane-containing gas stream 2 and steam stream3 are introduced into the tubular reactor 4, in which the treatment ofthe gas stream with steam takes place, with the formation of primaryhydrolysis products. The tubular reactor 4 dips into the coolingcontainer 5, which has a cooling jacket through which cooling agent 6flows. Upon entry of the gas stream into the cooling container, the flowrate of the gas stream greatly decreases. Steam condenses on the innerwalls of the cooling container, and the condensate, together withsecondary hydrolysis products, flows in the form of a film to the bottomof the cooling container, from which it is removed continuously orperiodically as a heterogeneous condensate 7. The residual gas 8 passesthrough the scrubber 9, which can be charged, for example, with sodiumhydroxide solution in order to remove hydrogen chloride and to monitorthe content of silicon-containing compounds. After passing through thescrubber, the residual gas leaves the system as waste gas 10. Generally,the waste gas is free of hydrogen chloride and of silicon compounds andcan be burned or used in some other way. The heterogeneous condensate 7can, for example, be dried at 100 to 160° C. in air in a paddle dryer(not shown in the figure) and the residue can be disposed of.

Second Embodiment

In a second embodiment of the process of the present invention, anotheraqueous liquid is employed as the liquid, aqueous phase for thetreatment of the gas stream instead of the condensate derived fromsteam. In this process, the optionally preheated gas stream generallyhas a temperature of up to 200° C., for example from 5 to 200° C.,preferably from 30 to 150° C., and the temperature of the other aqueousliquid is generally 10 to 90° C., in particular 20 to 50° C. Water vaporis present in the gas phase above the other aqueous liquid at a partialpressure of 12 to 700 mbar, depending on the temperature of this otheraqueous liquid, and serves as steam for the first stage of the treatmentof the gas stream, according to the present invention. This first stagetreatment is followed by the second stage of the treatment, in which thegas stream is treated with the other aqueous liquid.

The other aqueous liquid can be water, e.g. tap water, steam condensateor deionized water. Alternatively, it is possible to employ, forexample, an aqueous basic liquid, such as milk of lime, sodium hydroxidesolution or ammonia solution. If an aqueous basic liquid is used, thehydrogen chloride contained in the gas stream or formed by hydrolysis issimultaneously neutralized as the chlorosilanes are hydrolyzed. Inaddition, any Si—H bonds still present may react with the base to formSi—O groups (i.e., silanolates) and H₂.

Generally, the duration of the first and second stages of the treatmentlasts 5 to 120 sec., in particular 10 to 40 sec.

The other aspects of the equipment, process conditions, and theproperties and the treatment of the heterogeneous condensate are asdescribed above for the first embodiment of the process of the presentinvention.

FIG. 2 is a schematic diagram of an apparatus for carrying out thesecond embodiment of the process according to the present invention. Atthe mixing site 11, the chlorosilane-containing gas stream 12 isintroduced into the tubular reactor 13, whose inner walls are wettedwith “another aqueous liquid” 22, such as tap water or deionized water.The walls of the tubular reactor 13 are cooled by the cooling agent 14.Upon entry into the tubular reactor 13, the velocity of the gas stream12 decreases greatly. The treatment of the gas stream 12 with the steamoriginating from the other aqueous liquid 22 takes place in the tubularreactor 13, with formation of hydrogen chloride and primary hydrolysisproducts. The formation of secondary hydrolysis products takes place inthe other aqueous liquid 22 which wets the wall of the equipment,resulting in a suspension. This suspension is separated from thepurified gas stream 16 in the collecting container 15 and removedcontinuously or periodically as hydrolyzate suspension 15. The purifiedgas stream 16 then passes through the scrubber 17 which can be charged,for example with water, and leaves the system as waste gas 18. Scrubber17 may also be used for monitoring the content of residualsilicon-containing compounds in the gas stream. To re-use the otheraqueous liquid 22, the suspension can be recycled as a stream 19 untilthe concentration of the secondary hydrolysis products has reached apredetermined value. In addition, the circulation stream 19 can betreated to neutralize the hydrogen chloride formed by the hydrolysis ofchlorosilanes with an aqueous alkaline liquid 21, which can be removed,for example, from the reservoir 20. If the other aqueous liquid 22 orthe circulation stream 19 is rendered alkaline in this manner, largelysoluble silicon compounds result, thereby markedly reducing the tendencyfor deposits to form on tubular reactor 13 and collecting container 15.

Third Embodiment

The third embodiment of the process of the present invention is acombination of the two first embodiments. Thus, steam is employed forthe treatment of the gas stream in the gas phase in the first stage ofthe treatment, and additionally another aqueous liquid not exclusivelyproduced by condensation of the steam is employed for the second stageof treatment of the gas stream. The gas stream is thus treated with boththis “other aqueous liquid” and with the condensed steam. The “otheraqueous liquid” may be water or a basic, aqueous liquid. Alternatively,the heterogeneous condensate or its aqueous phase may be used byrecirculating a part of the liquid which collects at the bottom of thecooling container. It is possible, for example, to pass the “otheraqueous liquid” or the aqueous phase of the heterogeneous condensatethrough a ring line in the uppermost part of the cooling container 5 ofFIG. 1 on its cylindrical inner wall. This promotes the separation ofthe heterogeneous condensate as a result of cooling. In turn, the wastegas from the cooling vessel is free of silicon containing compounds andcan be burned or used in some other way.

Generally, the first and second stages of the treatment lasts 3 to 120sec., in particular 10 to 40 sec.

The other aspects of the equipment, process conditions, and theproperties and the treatment of the heterogeneous condensate are asdescribed above for the first embodiment of the process of the presentinvention.

FIG. 3 is a schematic diagram of an apparatus for carrying out the thirdembodiment of the process of the present invention. At the mixing site31, the chlorosilane-containing gas stream 32 and steam 33 areintroduced into the tubular reactor 34, where the treatment of thechlorosilane-containing gas stream 32 with steam 33 takes place withformation of primary hydrolysis products. The tubular reactor 34 dipsinto the cooling container 35, whose inner walls are wetted with the“other aqueous liquid” 44. In addition, the walls of tubular reactor 34can be cooled by means of a cooling agent 36. The flow rate of gasstreams 32 and 33 greatly decreases upon entry into the coolingcontainer 35. Steam 33 condenses in the interior of the coolingcontainer 35, and the condensate is transported to the wetted equipmentwall together with secondary hydrolysis products by means of a Stefanflow. The resulting suspension of secondary hydrolysis products,condensed steam 33 and other aqueous liquid 44 runs off the walls, iscollected in the collecting container 37 and removed continuously orperiodically. The purified residual gas 38 then passes through thescrubber 39, which may be charged, for example, with water, and leavesthe system as waste gas 40. In addition, scrubber 39 may be used tomonitoring the residual content of silicon-containing compounds in thegas stream. In the third embodiment of the process of the presentinvention, it is generally not necessary to remove hydrogen chloridefrom the waste gas 40. In order to minimize use of the “other aqueousliquid” 44, the suspension may be recycled as stream 41 until theconcentration of the secondary hydrolysis products has reached apredetermined value. In addition, the circulation stream 41 may betreated with an aqueous alkaline liquid 43, for example, from thereservoir 42, to neutralize the hydrogen chloride formed by hydrolysisof chlorosilanes. If the “other aqueous liquid” 44 or the circulationstream 41 is rendered alkaline in this manner, largely soluble siliconcompounds result, thereby markedly reducing the tendency deposits toform on cooling container 35 and collecting container 37.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only, and are not intended to belimiting unless otherwise specified.

EXAMPLES 1 TO 3

A glass apparatus according to FIG. 1 was used. The tubular reactor 4had a length of 2.0 cm and an inner diameter of 1.0 cm. The cylindricalcooling container 5 had an inner diameter of 140 cm and a volume of 4.5dm³. The temperature of the cooling water was 20° C., and had a flowrate of 12 l/min. The other experimental conditions were as shown inTable 1. The condensate containing the dissolved or suspended secondaryhydrolyzate was evaporated and the silica which remained as a residuewas dried at 150° C. and weighed. The results obtained are shown inTable 2.

TABLE 1 Waste gas steam 2 Tubular Cooling Trichlorosilane Steam stream 3reactor 4 vessel 5 Mixing Ex. Water flow (% by Temperature AmountQuantitative Residence Residence temp.* No. (g/h) (g/h) weight) (° C.)(g/h) ratio** time (sec) time (sec) (° C.) 1 9.0 9.9 52.4 160 548.255.2:1 0.01 213 129 2 9.0 9.9 52.4 220 750.0 75.8:1 0.01 213 154 3 18.09.9 35.5 220 657.5 66.4:1 0.005 107 152 *Treatment temperature; measured3 mm above the entry of the steam stream **Weight ratio of steam totrichlorosilane

TABLE 2 Silica (dried) Water Silica in gas Waste gas Amount content (%Waste gas 8 scrubber 9 10 amount Ex. No. (g/h) by weight) temperature(G/h) (g/h) 1 4.1 0 30 0.4 12.4 2 4.1 0 32 0.3 12.8 3 4.3 0 30 0.4 24.7

EXAMPLES 4 TO 6

The procedure corresponded to that of Examples 1 to 3, but using atubular reactor having a length of 80 cm and an inner diameter of 1.0 cmand a cylindrical cooling vessel having an inner diameter of 80 cm and avolume of 3.0 dm³. The other experimental conditions were as shown inTable 3 and the yields obtained are shown in Table 4.

TABLE 3 Waste gas steam 2 Tubular Cooling Trichlorosilane Steam stream 3reactor 4 vessel 5 Mixing Ex. Water flow (% by Temperature AmountQuantitative Residence Residence temp.* No. (g/h) (g/h) weight) (° C.)(g/h) ratio** time (sec) time (sec) (° C.) 4 18.0 9.9 35.5 220 642.064.8:1 0.5  84 155 5 13.5 9.9 42.3 220 468.9 47.2:1 0.6 108 151 6 9.09.9 52.4 220 562.9 56.7:1 0.9 162 155 *Treatment temperature; measured 3mm above the entry of the steam stream **Weight ratio of steam totrichlorosilane

TABLE 4 Silica (dried) Water Waste gas 8 Silica in gas Waste gas Amountcontent (% temperature scrubber 9 10 amount Ex. No. (g/h) by weight) (°C.) (g/h) (g/h) 4 8.1 47 25 1.0 23.0 5 7.7 46 25 0.9 17.3 6 6.9 40 250.9 11.5

EXAMPLES 7 TO 11

A glass apparatus according to FIG. 3 was used. The tubular reactor 34had a length of 1.5 cm and an inner diameter of 0.6 cm. The cylindricalcooling vessel had an inner diameter of 3.5 cm and a volume of 0.67 dm³.The temperature of the cooling water was 18° C., with a flow rate of 12l/min. The “other aqueous liquid” used was deionized water, which wassupplied at a pH of 12.5 in an amount of 70 to 100 l/h. The pH wasadjusted by means of 10% sodium hydroxide solution. The otherexperimental conditions were as shown in Table 5. The suspensioncontaining the secondary hydrolyzate was evaporated and the silica whichremained as a residue was dried at 150° C. and weighed. The resultsobtained are shown in Table 6.

TABLE 5 Waste gas steam 32 Tubular Cooling Trichlorosilane Steam stream33 reactor 34 vessel 35 Mixing Ex. Water flow (% by Temperature AmountQuantitative Residence Residence temp.* No. (g/h) (g/h) weight) (° C.)(g/h) ratio** time (sec) time (sec) (° C.) 7 9.0 9.2 58.5 220 468.935.7:1 0.001 20 165 8 13.5 9.9 42.3 220 138.9 14.1:1 0.005 19 138 9 18.010.7 37.3 220 468.9 43.7:1 0.001 11 155 10  9.0 12.3 57.8 — — — — 20 —11  18.0 6.3 25.9 — — — — 11 — *Treatment temperature; measured 3 mmabove the entry of the steam stream **Weight ratio of steam totrichlorosilane

TABLE 6 Silica (dried) Water Waste gas 38 Silica in gas Waste gas Amountcontent (% temperature scrubber 39 40 amount Ex. No. (g/h) by weight) (°C.) (g/h) (g/h) 7 6.6 38 30 0.8 12.4 8 6.8 38 30 0.3 16.9 9 7.2 38 300.2 24.7 10  7.7 31 30 0.4 12.4 11  3.7 28 40 0.4 12.4

The experiments of Examples 1 to 11 show that chlorosilanes are removedvirtually quantitatively. Even after 4 weeks' uninterrupted operation,the equipment showed virtually no coating on tubes, walls and otherequipment parts, let alone blockages.

The priority document of the present application, German patentapplication 19963433.5 filed Dec. 28, 1999, is incorporated herein byreference.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A continuous process for removing chlorosilanesfrom a gas stream, comprising: contacting a gas stream containing atleast one chlorosilane with water vapor in the gas phase in a firststage, and contacting said gas stream with a liquid, aqueous phase in asecond stage.
 2. The process of claim 1 wherein said water vapor issteam.
 3. The process of claim 1, wherein said gas stream comprises 3 to60% by weight of at least one chlorosilane.
 4. The process of claim 2,wherein said gas stream is heated to a temperature of at least 125° C.in said first stage with steam, and the liquid, aqueous phase of saidsecond stage is a heterogeneous condensate of water and secondaryhydrolysis products formed by cooling the gas stream below the dewpoint, or the aqueous phase of said heterogeneous condensate.
 5. Theprocess of claim 4, wherein the temperature of said first stage is 125to 250° C.
 6. The process of claim 4, wherein, in said first stage, saidgas stream is introduced into the interior of a stream of said steamsuch that the direction of flow of said gas stream and the direction offlow of said stream of steam are the same.
 7. The process of claim 4,wherein the weight ratio of said steam to said at least one chlorosilaneis 5:1 to 100:1.
 8. The process of claim 4, wherein, in the first stage,the mixture of said gas stream and said steam flows through a verticallyoriented reaction tube which projects centrally from above a coolingjacketed cylindrical cooling container into the lower third of saidcooling container in which said second stage takes place, and saidcooling container is cooled with flowing water.
 9. The process of claim4, wherein said first stage is carried out in a tubular reactor whichlies completely outside a cooling container, thereby forming a gasstream containing silica particles which is introduced tangentially intosaid cooling container.
 10. The process of claim 8, wherein said coolingcontainer is subdivided into two cooling zones.
 11. The process of claim9, wherein said cooling container is subdivided into two cooling zones.12. The process of claim 2, wherein the total treatment time of saidfirst and said second stages together is 3 to 300 sec.
 13. The processof claim 1, wherein the temperature of said gas stream is up to 200° C.14. The process of claim 1, wherein the temperature of said liquid,aqueous phase has a temperature of 10 to 90° C.
 15. The process of claim1, wherein the total treatment time of said first and said second stagestogether is 5 to 120 sec.
 16. The process of claim 1, wherein theliquid, aqueous phase is selected from the group consisting of water anda basic aqueous liquid.
 17. The process of claim 4, wherein in additionto said heterogeneous condensate of water or the aqueous phase of saidheterogenous condensate of water, said gas stream is contacted withanother aqueous liquid not produced by the condensation of said steam.18. The process of claim 17, wherein said another aqueous liquid isselected from the group consisting of water and a basic aqueous liquid.